The purpose of lime plaster, its advantages and disadvantages

For centuries, lime plaster has been utilized to provide walls with a sturdy and organic appearance. Because of its breathability, it’s highly regarded and a great option for older buildings where moisture control is important. Because of the way this plaster breathes, problems like mold and moisture are avoided.

Lime plaster’s flexibility is one of its key benefits. It is a durable alternative because, in contrast to contemporary plasters, it can absorb and release moisture without cracking. It’s crucial to keep in mind, though, that lime plaster takes longer to set than other kinds of plaster and requires more skill to apply.

Although lime plaster has numerous advantages, there are drawbacks as well. It can be more easily damaged by physical blows and is not as durable as plasters made of cement. Determining whether lime plaster is the best option for your project can be aided by being aware of its advantages and disadvantages.

Purpose of Lime Plaster	Advantages	Disadvantages
Provides a breathable surface	Natural and eco-friendly	Requires skilled application
Helps regulate indoor humidity	Resistant to mold and mildew	Can take longer to dry
Suitable for historic restoration	Durable and long-lasting	May be more expensive than other options

For centuries, lime plaster has been a versatile and environmentally sustainable building material. It provides natural breathability that helps avoid mold growth and moisture buildup while acting as a protective and decorative finish for walls. Its benefits include a pleasing, organic appearance, flexibility, and durability. To maintain its best appearance, lime plaster may need to be cleaned more frequently than other materials and can be more difficult to work with.

Peculiarities

For an extended period, lime has been produced by burning sedimentary rocks, which include chalk, shell rock, limestone, and marble that is not suitable for finishing. These rocks primarily consist of calcium carbonate and partially contain magnesium carbonate, with the percentage of each mineral varying depending on the deposit. As a result, lime produced from various deposits has varying quality. Fire turns these rocks into lime.

Lime can be: depending on how much clay is in the raw material

• airy, almost free of clay (<8%), hardening in air;
• hydraulic, containing 6-20% of clay particles, hardening under water.

The carbonates of calcium and magnesium break down during the firing process to produce oxides, which are the primary active ingredients in lime.

Lime arrives in various states at the construction site:

• lump quicklime – a product obtained directly as a result of firing;
• ground quicklime – a product of grinding lump lime;
• fluff – a product of slaking the two previous ones in the form of powder;
• dough (IT) – a paste-like mass, also a slaking product;
• milk – a suspension in which slaked lime is partially dissolved, partially in the form of a suspension.

Lime plaster is made by slaking quicklime to produce fluff, or lime dough, the benefits and drawbacks of which are discussed below. Slaked lime is referred to as lump, ground, and quicklime because of the high temperature that is released during the slaking process. Fluff is a component of ready-to-use dry mixes (RDM).

You should handle lime with caution because it is a caustic substance, especially quicklime. If lime gets in your eyes or on your skin, thoroughly rinse them with water.

Lime, in any form other than dough, is highly reactive, so it should be kept dry and away from moisture.

Solutions are prepared by adding ground quicklime to a concrete mixer; this causes the mixture to harden much faster and produces stronger coatings. Furthermore, because these solutions harden quickly and are heated by the heat released by the lime, they can be used at low temperatures.

Alkalis-resistant paints and dyes can be used to tint decorative lime plaster. Lime "eats" other people’s colors. The lime solution is tinted by adding no more than 5% (without running the risk of reducing strength) of pigments to produce the following colors:

• green (chromium oxide);
• yellow (ocher);
• brown (umber);
• red-brown (iron red lead);
• blue (ultramarine);
• black (soot).

Although light does not frighten paints, bright, saturated tones cannot be achieved.

Pros and cons

Lime plaster: benefits and drawbacks.

Since it is a mineral mixture, its primary benefits are as follows:

• ecologically friendly;
• fireproof;
• has a low price;
• adheres well to mineral bases, as well as to wood, plasterboard;
• vapor permeable (allows walls to breathe);
• repairable;
• durable;
• sufficient strength for wallpapering is achieved by lime plaster for interior work in about a day;
• easy to lay;
• bactericidal (does not allow the appearance of foci of fungus and mold).

The primary drawbacks

• long-term gain in strength (the coating holds nails after 2 weeks);
• low resistance to water;
• unstable – cracks appear when vibrating;
• needs to protect, as from strong alkali;
• not strong enough (inferior in this to others).

Scope of application of lime solutions

For the rough and final finishing of residential buildings, facades, and auxiliary buildings, the traditional building material is utilized. Lime mortar can be used to finish even moist areas like bathrooms with the addition of specific additives.

Since lime plasters are prone to cracking, they are not used in newly constructed buildings, areas with seismic properties, or near railroads until the building has finally shrunk. This is also the case with other mineral plasters.

There is also a better dry lime-carbonate plaster mixture for sale. By grinding limestone (60–70%) and boiling lime (30–40%) together, carbonate lime is produced. Because of this, compositions that use it solidify in humid environments and the air. Preparation calls for less water, and coating drying times are shortened. These mixes are meant to be used for making masonry mortars as well as lime mortar for plaster. The amount of water added varies: one liter is needed for plastering for every kilogram of dry mix, while 0.5–0.7 liters are needed for masonry.

Lime-gypsum mortars are stronger and harden faster—they can harden completely in 30 minutes—thanks to their enhanced properties. Their usage is more common in interior construction. They are also used for exterior coatings in low-humidity areas.

For home facades, lime-sand colored plasters are common; up to 20% of cement can occasionally be added. They are affordable and simple to use mechanically or manually.

Slaked lime, also known as hydrated lime, is a binder used in the lime construction mixture. These combinations serve a variety of functions and are even included in floor screed mortars used in construction.

Lime slaking

You will need a metal container to prepare a plaster solution with slaking lime because more heat and steam are released (up to 180 ° C) during the hydration process, which is a violent ongoing chemical reaction that results in a threefold increase in volume. Take the container with extra space in case the volume needs to be increased.

The quicklime packaging indicates the slaking speed, which can be:

• slow (> 25 min);
• fast (8-10 min);
• medium (15-20 min).

Throughout the process, a wooden paddle or shovel will be needed to mix the mass. Slaking is done for at least a day, regardless of the reaction rate. A lime that has been aged for one to one and a half months is the best.

It is recommended to wear protective clothing to avoid splashes.

1. Prepare a metal container in which to place the lump or powder limestone.
2. Prepare the required volume of cold water (to prepare lime milk, use 2 kg of ground limestone + 2 l of water, for dough for 1 kg of limestone – 0.5 l of water).
3. Slowly pour in (in small portions) water (this is especially necessary for lump limestone, so that it does not "coke").
4. As soon as the amount of steam generated decreases, add a little more water and stir carefully.
5. After steaming has stopped, cover the container with a lid and leave the mixture to rise for a day or a day and a half.
6. Stir again. Leave in a cool, dark place for at least 2 weeks.
7. After mixing, strain through a sieve.

How to prepare a lime mortar for plastering

Fat content is a distinguishing factor for the traditional lime mortar composition (with or without filler).

• very fat (recommended composition 1 part dough: 3.5-4 parts sand);
• fat (1:2.5-3) is often used for a cover layer (1:1), putty-free plastering (sand fraction <0.5 mm);
• medium fat (used in compositions 1:2-2.5);
• lean (1:1.5-2);
• very lean (even less sand – 1:1-1.5).

Use fully slaked lime for plastering; otherwise, the coating will swell and flake off. Add it to the mixture in the form of water-diluted dough or milk. Lime mortar can be prepared two to three days in advance of work because lime plasters set slowly.

Lime-gypsum mortar

Lime-gypsum plaster is created by incorporating gypsum into the plaster mixture. This significantly expedites the finishing process.

Take the prepared lime mortar (three to four parts) and the gypsum (one part) for preparation. Put two liters of dough into a mixing bowl.

Add one liter of gypsum, blend with a mixer, and then dilute with water. Next, add two more liters of dough and stir everything together.

Stir in small amounts of gypsum powder to the lime milk to create a sand-free cover. Maximum layer of cover: 2 mm. Make-up of the plaster cover mortar (gypsum – IT):

• dry – 1:1;
• slightly damp – 1:2;
• wet – 1:3.

Using a metal or rubber trowel, smooth the cover without wetting it.

Because the setting begins five minutes after the layer is mixed, the mixing is completed quickly, the layer is fully processed before the setting begins, and the mixing volume is kept to a minimum.

Add 10 milliliters of PVA glue to one liter of plaster mix to extend its life.

Lime-clay mortar

Plaster can be filled with clay to cut down on the amount of lime needed, and clay can be strengthened with boiling water to make it more resistant to water erosion. In both situations, the additive’s volume may increase to 35%. Before slaking, these ingredients are combined in a metal container or tvoril (pit), filling them in layers. After adding water, let it sit for at least ten days. Then, blend, strain, and, if required, add filler in the appropriate amount to the composite dough.

Typically, these compositions are utilized for interior design. Ratio (filler: dough):

• for splashing – 1:1 (for wooden walls, the consistency of the solution is semi-thick, for concrete/stone – more liquid);
• for the primer (base layer) – 1:0.75:0.5 (the last figure for sawdust or other fibrous additives);
• for the top coat – 1:1 (dough: sand).

Plaster uses fibrous fillers to boost the low strength of these kinds of solutions. It is best to soak the fibrous fillers in a 5% solution of copper sulfate before adding.

Another method is to prepare the clay dough and lime dough separately. The latter is done ahead of time by adding water to a different container and letting it sit for one to two days. After that, knead the clay dough and combine it with lime after draining the water that the clay did not absorb. Next, make a clay-lime plaster mortar by adding water and sand while keeping an eye on the amount of fat in it. Add lime if it doesn’t adhere to the tool.

Lime-cement mortar

You can make a plastic cement-lime plaster mortar (CIM) that is durable and all-purpose by hand or purchase a ready-made dry mixture (DM) that you can dilute with water to the proper amount. Nevertheless, SS are more costly than independently prepared compositions. As a result, we offer information to people who desire to perform manual labor.

The composition and ratio of the components may change depending on the solution’s intended use. The most common uses of cement are grades 400–500 (white; decorative); the medium fraction is filled with washed sand, which can be quartz, river, quarry, or ravine sand.

The component ratios in cement-lime mortar compositions for plastering under various operating conditions vary:

• in bathrooms, basements, cellars, where there is increased moisture condensation, – 1: 2: 8 (volume of cement powder M400 – dough – sand);
• on smooth external surfaces – 1: 1-3: 6-9;
• surfaces that are systematically moistened and exposed to frost (foundation, basement area) – 1: 0.5-1: 4.5-6.

After combining the dry ingredients (sand and cement powder), milk is added.

Lime-sand mortar

Take 2.5 to 4 parts sand to 1 part dough to make the initial lime-sand plaster. The finishing layer has a different composition. It follows the formula 1: 1-2 (see above). Sand can be added to dough that has been diluted with water or combined with milk. As the lime-sand plaster dries, its gray hue will turn to a snow-white hue.

Furthermore, the form of the sand particles affects the coating’s strength. Because of its fractured, crushed stone-like shape and improved adhesion to lime, ravine sand—which is extracted from mountain quarries—has a stronger coating. Because the surface of sea and river sand is rounded, it adheres less. This type of sand needs to be cleaned because it may also contain silt-clay particles, which weaken it.

Additional components

Using different kinds of filler results in an improved composition, for instance, for decorative plaster.

• stone flour (Venetian plaster);
• perlite, expanded clay, expanded polystyrene (heat-insulating, noise-absorbing);
• marble or other rock chips (decorative compositions);
• artificial fibers (fiber) or natural (sawdust, wool, chaff), increasing strength and reducing heat transfer;
• barite sand (protective mixture used in X-ray rooms);
• crushed mica (increases weather resistance and water resistance, reduces static, protects against UV rays).

Fibrous materials are used to reinforce plaster for external work on a reinforcing mesh (wool waste, finely plucked felt, etc.). The coating is stronger and the mesh cells are better filled by this composition. Additive volume: 0.5–1 parts for each dough portion.

Preparing the working surface

Correct base preparation is essential for the plaster finish to function over time while retaining its strength and beauty.

In the order listed below, prepare the surfaces for applying lime mortar plaster to the walls:

• we remove the wallpaper, inspect the wall to identify defects and weak points, tap the old plaster coatings (if left), knock off the peeling areas;
• neutralize areas affected by mold and mildew;
• remove oil stains, rusty deposits, efflorescence;
• remove or isolate metal objects;
• we repair chips, cracks (for large ones we use a reinforcing mesh);
• remove dust;
• prime with a composition corresponding to the base and finishing material;
• install beacons;
• dry.

Tools

Both making and applying the lime mortar will require the following supplies:

• ladle;
• rule;
• grater;
• trowel;
• spatula;
• paint brush;
• containers;
• mixer.

Application technology

The outcome of using lime plaster

Three layers of the lime mortar are applied, each with distinct qualities:

1. Do not skip the work described in the section on preparing the walls
2. The first, called splash, is done to ensure reliable adhesion of the finish to the base. It has the consistency of thin sour cream. It is thrown on with a ladle or trowel. It is not smoothed out.
3. The primer (the main layer is 10-30 mm thick) is made with a more viscous composition using a trowel between the beacons. If it is necessary to apply a thicker coating, the composition is applied layer by layer with a daily drying of each layer. Align it along the beacons with a rule. After the setting begins, the installed beacons are dismantled, and the resulting grooves are filled with mortar.
4. The cover (the finishing layer, which usually has a different composition, the consistency of the splash) is applied with a spatula or a trowel several mm thick.

The splash and cover are composed differently from the primer. A lime-cement mortar splash "asks" to be added to a concrete wall. Covering is another use for it in facade plastering. It is better to cover rooms with a lime-gypsum mortar.

A timeless and adaptable material, lime plaster has withstood the test of time. Because of its inherent qualities, it’s a great material to use to create long-lasting, breathable surfaces. It is especially prized for its capacity to control humidity, which can improve indoor air quality.

Lime plaster has certain disadvantages despite its many advantages, including sustainability, flexibility, and ease of repair. Applying it correctly takes skill and patience, and the curing process can take a while. Lime plaster, however, can be a satisfying option for both historical restoration and contemporary construction for those prepared to put in the time and work.

Lime plaster may or may not be the best option for your project, depending on your unique requirements and the characteristics that are most important to you in a building material. Its unique combination of challenges and natural benefits makes it a worthwhile option.

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